The present invention is related to apparatus for end joining successive board lengths to form a single elongated piece of lumber, and more particularly to such devices for end joining individual boards having parallel fingers cut or pressed into abutting opposed transverse ends thereof.
There are two basic forms of apparatus used for end gluing and finger jointing short pieces of wood in end-to-end relationships to make long length lumber. The first is distinguished by a "stop and go" assembly procedure, while the second makes use of a continuous assembly.
"Stop and go" assembly may involve apparatus for aligning short pieces of wood spaced several inches apart from one another. Adjacent pieces of wood are then held in rigid clamps during the machining of a fingered configuration in the opposing ends by use of tapered saws or cutter heads. The ends can also be formed by pressing them on a die. The fingered configurations of the ends are then coated with an adhesive and clamped. The adjacent pieces are subsequently moved (by moving one or both of the clamps) toward each other to mate or close the joint. Sufficient pressure is applied during the jointing step to obtain a good glue bond at the mating surfaces of the fingers. Alternatively, the finger configurations are first brought together and then clamped only during the curing time of the adhesive.
The main disadvantage with "stop and go" end gluing is that a considerable amount of time is required before the adhesive within the joint will cure sufficiently. Because the wood is basically stationary during the clamping time, "stop and go" assembly is necessarily characterized by unacceptably low volume production.
On the other hand, a primary advantage with the clamping "stop and go" method is that the wood pieces can be held in perfect alignment during machining and mating. Misalignment of the joint can usually be prevented, even if the wood is warped, bowed, cupped, twisted, or otherwise moderately crooked. The product is therefore straight, since offsets at the joint or other misalignment problems rarely occur. Another advantage of the "stop and go" apparatus is that very small, short fingers can be used in the joint due to the accurate control maintained during the joint mating step.
Apparatus for continuously assembling finger joints produces a continuous flow of wood pieces through various types of conveyors during the mating, end pressing and glue curing steps. The conveyors mate the end finger configurations of longitudinally moving pieces and then apply end pressure necessary for glue bonding by varying the speed and longitudinal movement of successive boards along the operating conveying surfaces. Cure of the adhesive is usually accomplished by continuing to move the newly mated joints through a radio frequency field. With such procedures, the machining of fingers and application of glue to the end surfaces precedes the mating step and is accomplished by unrelated machinery.
The main advantage demonstrated by continuous flow finger jointing apparatus is high speed, high volume production. However, the desirable features of accurate alignment and capability of operating with short fingers exhibited by the "stop and go" apparatus is sacrificed. Warp of any kind in the wood tends to cause misalignment of the joint. Offset at the joint, whether it be horizontal or vertical, frequently results in a joint being mismatched by one finger, crossover of a finger from one groove to another, or two fingers entering one groove. The result is a weak or defective joint.
The difficulty encountered in accurately mating finger joints by use of a continuous assembly apparatus is partially overcome by using joint designs that make use of relatively long fingers and wide finger "pitch" (the distance from one finger tip to the tip of an adjacent finger). The greater pitch creates a wider gap into which a mating finger tip can enter. However, long, wide pitch fingers are a disadvantage in that they waste valuable wood product removed by machining. Long fingers also tend to split more readily when high gluing end pressure is applied.
Another disadvantage encountered with continuous flow apparatus is that pressure is not applied in a direction perpendicular to the outermost fingers. The wedging action of the mating "V" shaped fingers tends to prevent good contact of the outer fingers as the glue cures. The result is frequent poor glue bonds between the outermost fingers and their mating surfaces. Poor glue joints between long finger joint designs represent a significant portion of the cross section of the finger jointed member and therefore substantially weaken the resulting board at the glue joint, particularly in the zone of highest bending stress concentration. If the outermost fingers are not properly bonded, especially in long joint designs, the result is a weak, non-structural end joint that cannot qualify for use as higher grades of structural lumber.
The present invention overcomes the main disadvantages of both types of existing apparatus for assembling finger jointed lumber. The apparatus disclosed herein makes use of a semicontinuous flow, combining advantages of precise joint mating and high volume production. It accepts boards for bonding in a system in which the machining of fingers and the application of glue have previously been carried out by conventional means.